Aluminum oxide is a starting material for manufacturing ceramics having particularly high hardness and good chemical corrosion resistance. Aluminum oxide ceramic is customarily manufactured by sintering mixtures of aluminum oxide and other oxides. Extremely high-quality formed parts of aluminum oxide ceramic are produced from practically pure aluminum oxide, to which may be added, as a sintering adjuvant, small fractions of MgO or TiO2, such that the resulting ceramic has an Al.sub.2 O.sub.3 content of 99.7%. Metallurgical grade Al.sub.2 O.sub.3, produced by the Bayer-Process from bauxite, is typically unsuitable as a starting material for the manufacture of aluminum oxide ceramic, due to interfering impurities such as Na.sub.2 O--approx. 0.3-0.5%; SiO.sub.2 - approx. 0.02-0.05%; Fe.sub.2 O.sub.3 - approx. 0.02-0.05%. Further, metallurgical grade aluminum oxide is too coarse for producing high quality ceramic, having a mediate grain diameter of about 50-100 um. To provide the desired properties in the ceramic, the aluminum oxide must possess uniform characteristics achieved from a relatively precise degree of calcination, combined with proper grinding These characteristics of the aluminum oxide are discussed further below.
For certain applications, a transparent aluminum oxide ceramic having specific characteristics and prepared according to specific methods is required. Transparent aluminum oxide ceramic is particularly useful in the construction of sodium vapor high-pressure electric lamp tubes, because aluminum oxide ceramic provides a high chemical resistance to the aggressive vapor.
The manufacture of transparent aluminum oxide ceramic requires an aluminum oxide of particularly high chemical purity. In particular, the concentration of SiO.sub.2, Fe.sub.2 O.sub.3, Mn.sub.2 O.sub.3 and other heavy metal oxide impurities in the aluminum oxide, is critical in determining the suitability of the aluminum oxide for the manufacture of transparent ceramic. The reason for this is that the heavy metal oxides and silicon oxides are reduced by sodium vapor under the operating conditions of the sodium vapor high-pressure lamps, resulting in the formation of elemental metal, which in turn leads to a reduction in light transmissibility of the ceramic lamp tube.
A particular difficulty in the manufacture of transparent ceramic is that a sufficiently pure aluminum oxide starting material must be obtained. One known method for obtaining purified aluminum oxide proceeds by way of thermal degradation of ammonium alum, AlNH.sub.4 (SO.sub.4).sub.2. Ammonium alum can be purified easily because of its particularly good crystallization properties, resulting in a low impurities content. Other methods of purifying aluminum oxide are based on the fractionation/distillation of volatile aluminum compounds, such as aluminum alkoxide and aluminum alkyl, which can be subsequently degraded to aluminum oxides or its precursors, such as aluminum hydroxide or other aluminum salts.
Both methods generally provide acceptably low levels of impurities, as illustrated by the following limits:
______________________________________ SiO2 max 20 ppm (parts per million) Fe2O3 max 10 ppm Cr2O3 max 5 ppm TiO2 max 10 ppm other elements max 5 ppm ______________________________________
The existing technology for the manufacture of transparent ceramic lamp tubes requires highly calcined aluminum oxides in order to attain the necessary properties required for further ceramic processing. In particular, calcination provides a BET surface area of 1-10 m.sup.2 /g (BET = nitrogen absorption according to Brunauer, Emmet & Teller (see ISO Standard, Ref.No.ISO-8008-1986(E)).
The calcination of high-purity oxides is fraught with problems, since, when direct firing is used, impurities can be introduced into the product by the fuel used to produce the heat for calcination. Heating indirectly does avoid this contamination source. However, indirect heating is not readily adaptable to continuous operation, and during batch calcination, the danger exists that, due to the low powder density producing poor heat conductivity, a temperature gradient will occur across the material, creating large differences in the calcination state of the oxide. The calcination state determines the ceramic reactivity of the oxide, and consequently, to obtain uniform sintering, there should be only minor variations in the calcination state. Hence, the processing properties of the calcinated material is not generally acceptable for transparent aluminum ceramic manufacture.
An additional critical aspect of oxide manufacture for use in transparent ceramics is the average particle size of the calcined oxides. The calcined oxides must be ground extremely fine, for the oxides to be subsequently successfully sintered, such that dense formed bodies can be obtained. If properly calcined material is obtained, it is generally hard, requiring contact grinding which presents the danger of contamination of the oxide material by particles abraded from the grinding substance. The contaminating particles can create faults in the sintered product and, consequently, increase the rate of rejects in the manufactured items of transparent ceramic. Therefore, the critical difficulties in the preparation of aluminum oxides for transparent ceramic lies in the calcination and grinding steps of the pure aluminum oxide.
It is known that transparent bodies can be created by sintering aluminum oxide monohydrate gel. However, these bodies are also known to have a high degree of porosity, which precludes their use as lamp tubes, since they are not gas-impermeable.
B. E. Yoldas, [Amer. Ceram. Soc. Buil. 54 (1975) 286 and Journal of Materials Science 10 (1975) 1856], has described that by the pyrolization of formed bodies, produced of aluminum oxide monohydrate gel, transparent but highly porous bodies are formed which may be used as catalyst carriers.
The method developed by Yoldas has the disadvantage that the gel is formed into bodies, which are subsequently dried. During the drying process, there is a danger that tears might appear in the formed bodies due to shrinkage stresses. The tears formed in the bodies during drying do not heal in the calcination process. Accordingly, the method is suitable only for the manufacture of thin layers, for placement on carriers of different types, such as aluminum oxide or another ceramic material. Consequently, the search continues for compositions and methods for producing transparent ceramic while avoiding the known consistency and porosity problems.